1. Field of the Invention
The present invention relates to an intensity control unit for controlling intensity of light emitted from a light source such as a semiconductor laser and to an image forming apparatus using such an intensity control unit.
2. Description of the Related Art
An electrophotography-type color laser printer or the like employs a semiconductor laser as a means for forming a electrostatic latent image on a photosensitive drum according to an image signal.
FIG. 5 shows an overall structure of an example of a single-drum multi-transfer color laser printer. In FIG. 5, paper 102 supplied from a paper feeder 101 has a tip thereof gripped by a griper 103f of a transfer drum 103 and is retained along the outer circumference of the transfer drum 103.
Latent images of different colors formed on a photosensitive drum 1 by an optical unit 107 are developed by color developing mechanisms Dy, Dc, Dm, and Db, respectively from yellow, cyan, magenta, and black coloring agents, and thereafter transferred onto paper wound around the outer circumference of the transfer drum 103. Thus, a multicolor image is produced. Thereafter, the paper 102 is separated from the transfer drum 103, fused by a fusing unit 104, and ejected to a paper discharge tray 106 by an ejector 105.
Each of the color developing mechanisms is supported for rotation an axis of rotation, and all of the color developing mechanisms are held by a developing mechanism selector 108 that can rotate about an axis of rotation 110 which is fixed to a housing of the printer. As shown in FIG. 5, as the developing mechanism selector 108 rotates for sequentially selecting each developing mechanism in turn, the developing mechanisms rotate around their respective axes so as to maintain a constant vertical orientation.
After a selected developing mechanism has been moved to a developing position, a selector holding frame 109 is pivoted toward the photosensitive drum around vertex 109b by means of a solenoid 109a, thereby positioning the developing mechanism selector 108 together with the developing mechanisms. A detector 4 detects a home position of the developing mechanism selector 108 by detecting a projection H projecting from the circumference of the developing mechanism selector 108.
A printer control unit that is not shown recognizes the positions of the color developing mechanisms using the projection H as a reference, determines an angle of rotation to be made by the developing mechanism selector 108, and then selects a desired developing mechanism.
For image formation lighting in the printer having the foregoing structure, light emanating from a laser beam scanner is utilized to form latent images on photosensitive drum 1.
An image formation lighting means shown in FIG. 6 is an example incorporated in the optical unit 107 shown in FIG. 5. A laser beam scanner serving as an image exposing means has a semiconductor laser 41. Light emanating from the semiconductor laser 41 is modulated according to color components of an image signal. A laser beam emitted by the semiconductor laser 41 is deflected by a rotary polygon mirror 38 via a collimator lens 31 and a cylindrical lens 32, imaged by an f-.theta. lens composed of a spherical lens 33 and a toric lens 34, turned back by a reflector mirror 35, and then irradiated to the photosensitive drum 1. The laser beam is then scanned over the surface of the photosensitive drum 1 in a given direction (indicated by arrow "a") at a certain speed. Thus, an image is produced according to the color components of the image signal. Part of the laser beam is reflected by a horizontal synchronization mirror 36. An index signal (sometimes called a "beam detect" or "BD" signal) indicating a start direction of a scanning line made by the laser beam is detected through optical fibers 37. Using the signal as a reference, timing of writing an image along one scanning line is determined.
FIG. 7 is a block diagram concerning intensity control executed by a laser drive circuit employed in the optical unit 107. In FIG. 7, reference numeral 41 denotes a semiconductor laser identical to the one shown in FIG. 6. 40 denotes a laser unit, which is composed of the semiconductor laser 41 and a photodiode 42 located in the vicinity of the semiconductor laser 41. Part of the laser beam emanating from the semiconductor laser 41 is received by the photodiode 42, thus providing a signal S40 whose level is proportional to the intensity of the laser beam. Next, the signal S40 is compared in signal level with a reference signal Vt representing a predetermined target value of intensity.
Then, an output signal S44 of a signal comparator 44 is supplied as a signal whose level corresponds to an error in level between the signals S40 and Vt. The control circuit 45 supplies a counting control signal S45 to a counter 46 according to the error signal S44.
Next, the counter 46 counts up or down pulses in response to the control signal S45, and supplies a count signal S46 to a D/A converter 47. The D/A converter 47 converts the digital signal S46 into an analog signal S47. The signal S47 is then supplied to a laser drive circuit 48 (which is also depicted in FIG. 6). The laser drive circuit 48 also receives a signal S1. The signal S1 is an image signal supplied from an image signal control unit that is not shown.
The laser drive circuit 48 specifies the intensity of a laser beam according to the signal S47, generates a laser drive signal S48, which controls the on or off operation of a laser, using the image signal S1, and supplies the signal S48 to the semiconductor laser 41.
The above process of stabilizing the intensity of light emanating from the semiconductor laser 41 and confining it to a given range is referred to as automatic power control which shall be written hereinafter as "APC".
A circuit for stabilizing the intensity of light emanating from a laser is incorporated in the optical unit 107, because the temperature characteristic of the laser is very poor. In an environment of a varying ambient temperature, APC is essential for stabilizing the intensity of light emanating from a laser.
A direction in which a laser beam scans the photosensitive drum 1 (indicated with arrow "a" in FIG. 6) is ordinarily called a "main scan" direction and a direction in which the photosensitive drum 1 rotates (indicated with arrow "b" in FIG. 6) is ordinarily called a "sub scan" direction. APC is carried out in non-print (or imaging) areas in the main and sub scan directions. More particularly, the non-print periods during which the non-print areas are scanned are classified into pre-processing periods each preceding initial image formation, vertical blanking periods each occurring between sheets of print paper (also between colors in multicolor printing mode), and horizontal blanking periods each occurring between print lines.
Ideally, APC should be carried out over all the non-print areas. During APC, the semiconductor laser 41 is forced to emit light. For a longer service life of the laser, APC is executed during any time interval within a non-imaging period which is dependent on a printer.
APC to be performed during the pre-processing period shall be referred to as initial APC. APC to be performed during the horizontal blanking (hereinafter, referred to as unblanking) period occurring between sheets of print paper (also between colors) shall be referred to as unblanking APC.
FIG. 8 shows timing of executing unblanking APC. In FIG. 8, a TOP signal is a signal to be sampled every time the transfer drum 103 rotates to a given position. The TOP signal serves as a vertical-sync signal for an image signal. A BD signal is a reference signal indicating a start line of writing print paper. The BD signal serves as a horizontal-sync signal for the image signal. An UBL signal is a signal that becomes active during an unblanking period alone within main scanning. When the UBL signal is active, unblanking APC is executed. Unblanking APC is executed during the unblanking (horizontal blanking) period within a T2 time interval in FIG. 8.
Next, referring to FIG. 7, actual APC will be described in terms of a non-print period and a print period.
To begin with, during a non-print period, the semiconductor laser 41 is turned on forcibly for a given APC period. Control is then passed as mentioned below. A signal S40 whose level is proportional to the intensity of light emanating from the laser which is detected by the photodiode 42 is fed to the signal comparator 44, and compared with a reference signal Vt representing a target value of intensity. The subsequent operation depends on the result of comparison.
Specifically, if the result of comparison reveals that the detected intensity equals to the target value, a request signal S44 requesting for retention of a current laser drive signal is supplied to the control circuit 45 in order to latch the output signal S47 (current state) of the D/A converter 47. On the other hand, if the result of comparison reveals that the detected intensity does not equal the target value, a request signal S44 requesting for increase or decrease, as appropriate, of the intensity is supplied to the control circuit 45. For example, when the detected intensity exceeds the target value, a request signal S44 requesting for decrease of the intensity of light is supplied to the control circuit 45. In response to a control signal S45 sent from the control circuit 45 to the counter 46, the counter 46 counts up or down pulses and supplies a count signal S46 composed of a plurality of bits to the D/A converter 47. The D/A converter 47 then converts the input signal S46 into a laser drive signal S47 of analog form. The laser drive signal S47 is then supplied to the laser drive circuit 48, thus changing the intensity of light emanating from the semiconductor laser 41. Thereafter, the foregoing control sequence is repeated until the detected signal S40 sent from the photodiode 42 approaches in level the reference signal Vt representing a target value of intensity.
During a print period, the control circuit 45 instructs the counter 46 to stop counting and supplies a control signal S45 so as to trigger latching. Thus, the laser drive signal S48 confined to a certain range by performing APC during a non-print period is retained. During the print period, the intensity of light emanating from a laser is therefore held at a value attained by performing APC immediately before printing. This results in stable print density.
However, there are differences in measurement conditions between when initial APC is performed and when unblanking APC is performed. These differences result in a different error derived from the different APC's.
(1) First, there is a difference in forced light emission time between initial APC and unblanking APC, which causes a difference in stability of the intensity of light emanating from a laser.
That is, during initial APC, light is emitted forcibly throughout at least one main scanning line including an effective print area in order to search for a reference signal indicating a start line of writing print paper (BD signal). In contrast, during unblanking APC, since the BD signal is already detected, light is emitted forcibly only for a given period of time (horizontal unblanking period) preceding or succeeding the BD signal.
Unblanking APC involves several main scanning lines until APC is completed. Light is emitted forcibly during the horizontal unblanking period alone. During a sub scan period, therefore, forced light emission is repeated pulsatively. Thus, unblanking APC differs from initial APC in which light is emitted forcibly during both the main and sub scan periods. FIGS. 9(a) and 9(b) show these relationships. A period during which an UBL signal is active low corresponds to a period during which a laser is forced to emit light due to APC. FIG. 9(a) shows timing of executing a sub scan. FIG. 9(b) shows timing of executing a main scan. A period during which a masking signal is active (or "high") corresponds to a period during which an effective print area is scanned.
(2) Second, there is a difference in non-print time required for starting subsequent printing between initial APC and unblanking APC.
That is, it takes a longer time (FIG. 9(a)) interval to start subsequent printing after initial APC. There is a higher probability that the intensity of light emanating from a laser for printing varies after completion of APC.
(3) Third, initial APC starts in a state in which a laser has not emitted light at all; that is, a laser drive control signal S46 is cleared. On the contrary, unblanking APC starts with a count value attained at the end of the previous APC.
This relationship is illustrated in FIGS. 10(a) through 10(c). In FIG. 10(c), image formation for the first page starts from the end of initial APC to the starting time of period TC1, TC2, and unblianking APC takes place in appropriate intervals during TC1, TC2.
In general, during initial APC, as shown in FIG. 10(a), coarse adjustment (intensity control plotted as A in FIG. 10(a)) in which a count value is changed by several bits and fine adjustment (intensity control plotted as B in FIG. 10(a)) in which a count value is changed by one bit alone are combined in order to reduce initial APC time. Specifically, coarse adjustment is carried out until the intensity of light emanating from a laser reaches a given value close to a target value of intensity represented by the reference signal Vt. Fine adjustment is then carried out until the intensity changes from the given value to the target value. In contrast, since unblianking APC is started with the intensity of light emanating from a laser attained at the end of the previous APC, fine adjustment alone is carried out. Thus, initial APC and unblanking APC use different APC tracking techniques.
When initial APC is achieved, as shown in FIG. 10(b), by performing fine adjustment alone, the same technique as the one followed by unblanking APC can be employed as shown in FIG. 10(c). However, an initial APC period Tb is longer than an initial APC period Ta as shown in FIGS. 10(a) and 10(b). Thus, initial APC including fine adjustment alone is impractical because of the long APC period. Besides, an image signal to be printed is supplied with a time lag. This causes an operator to wait for a prolonged period of time.
(4) Fourth, no consideration is taken into a droop characteristic (described below) of a laser. Therefore, since there is a difference in control sequence between initial APC and unblanking APC as described in items (1) to (3), even if light emission current to be fed to a laser element at the completion of APC is the same between initial APC and unblanking APC and light is emitted with the measured current, the intensity of light at the completion of APC differs between initial APC and unblanking APC.
"Droop characteristic" refers to the fact that, when a semiconductor laser is driven, the temperature at a junction in a laser element varies with the intensity and duration of light due to transitional heat resistance in the laser element. That is to say, when light is emitted initially, the temperature at a junction is low and the efficiency in light emission is therefore excellent. With an increase in the temperature at a junction due to self-generated heat, the efficiency in light emission deteriorates.
When initial APC is performed or light is emitted continuously, the efficiency in light emission is poorer than that when unblanking APC is performed or light is emitted for a short period of time. The intensity of light therefore differs between initial APC and unblanking APC despite the same drive current. As a result, even if the laser drive current is set for initial APC and unblanking APC in order to attain the same target intensity of light, the intensity of light for forming an image differs between initial APC and unblanking APC.
FIG. 11 shows timing of executing initial APC and unblanking APC for a multi-rotation color printer. In FIG. 11, T1 denotes an effective print period during which one sheet of print paper is printed. T2 denotes a vertical blanking period occurring between colors. T3 denotes an unblanking APC period. T4 denotes an initial APC period. VO denotes a target value of intensity of light emanating from a laser. Vt denotes a reference signal for use in controlling the intensity of light emanating from a laser. Delta A (".DELTA.A") denotes an error derived from initial APC. Delta B (".DELTA.B") denotes an error derived from unblanking APC.
Because of the aforesaid factors, there arises a problem that a difference is made in print density between the first sheet of paper (initial APC) and a subsequent sheet (unblanking APC). Furthermore, when a multi-rotation type color printer is employed, as shown in FIG. 11, a difference is made in density between the first color and the second color in one sheet of print paper.